One of the most powerful natural explosions ever observed on Earth occurred in 1908 when a comet or a meteor exploded above the Tunguska River in Siberia, Russia. The Tunguska event destroyed trees more than 25 miles away and caused a powerful earthquake. The above picture was taken almost 20 years after the event. Trees cover the ground like toothpicks. Estimates of the object's size range from 200 feet to more than 3,000 feet in diameter.